Human T-cell leukemia virus type I (HTLV-I) and bovine leukemia virus (BLV) are related retroviruses associated with lymphoid malignancies in humans and cows, respectively. The biology of these viruses has been difficult to study due to their highly restricted expression; such phenomena as receptor-binding, RNA packaging signals, effects of mutations of structural genes, infectivity, etc., have not yet been addressed. To facilitate the study of these viruses, we have begun to generate recombinant viruses containing foreign genes controlled by strong promoters. Thus, the virus infectious cycle can be easily monitored by following the expression of the reporter gene product. We have already constructed a self-packaging, recombinant BLV which contains the NEO-resistance marker and requires complementation with viral regulatory genes only. In addition, a recombinant HTLV-I provirus was made containing the NEO-resistance marker that must be complemented with viral env and regulatory gene products. These recombinant viruses have several drawbacks for experimental use: first, cell clones expressing the selectable marker, NEO, take several weeks to identify by drug selection; second, the recombinant virus titers obtained have been low compared to parental virus. To overcome some of these problems, we have begun to construct a packaging-deficient HTLV-I helper virus to supply, in trans, the genes required for expression and packaging of the recombinant virus. Also, a new HTLV-I recombinant provirus containing the bacterial lacZ gene is being constructed; this will allow identification of cells expressing the lacZ gene in days rather than weeks.